1. Field of the Invention
The invention relates to methods of producing magnetoresistive elements, and more particularly, to methods of producing magnetoresistive elements having an induced secondary anisotropy.
2. Description of the Related Art
Storage memories of various types are used extensively in digital systems such as microprocessor-based systems, digital processing systems, and the like. Recently, magnetic random access memory (MRAM) devices have been investigated for possible use in non-volatile random access memory. Information is stored in MRAM devices based on a magnetoresistive effect, in which memory cells formed of ferromagnetic layers in the device have resistances that change based on the magnetized state of a free ferromagnetic layer compared to that of a pinned (fixed) ferromagnetic layer. The magnetic moment of the pinned layer remains fixed while the magnetic moment of the free layer can change depending on an externally-applied magnetic field or potential. The relative magnetic directions of the free layer to the pinned layer typically are referred to as “parallel” and “antiparallel.”
A magnetic memory element, such as a magnetic tunnel junction (MTJ), is formed on a wafer substrate. The structure includes free and pinned ferromagnetic layers separated by a non-magnetic tunnel junction barrier. The magnetic memory elements are formed using thin-film materials and can be manufactured on the sub-micron level.
In response to parallel and antiparallel magnetic states, the magnetic memory element represents two different resistances to a current provided across the memory element in a direction perpendicular to the plane of the ferromagnetic layers. The tunnel barrier is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs across the barrier junction between the two separated sets of ferromagnetic layers. The resistance across the element has minimum and maximum values corresponding to whether the magnetization vectors of the free and pinned layers are parallel or antiparallel.
Consequently, it is necessary when producing magnetic memory elements to provide layers having magnetic moments that are selectively aligned. Magnetic memory element structures include very thin layers, also known as ultrathin films, some of which are tens of angstroms or less in thickness. The structure of ultrathin films has a strong influence on their magnetic properties. Small variations in thickness and surface morphology can impact the magnetic characteristics of an ultrathin magnetic film layer.
Magnetic anisotropy, the tendency of the magnetic moments in the layer to align in a given direction, can be influenced by the shape of the layer. Magnetic moments tend to align head to tail, rather than head to head, so by forming a layer as a rectangle, for example, the magnetic moments will tend to align parallel to the longer dimension of the layer. This phenomenon is known as shape-induced anisotropy. It would be advantageous to be able to induce anisotropy without regard to the overall shape of the film layer.
It is known in the prior art to form layer shapes using photolithography. Photolithography is susceptible, however, to image distortion and instabilities, problems that are exacerbated at the sub-micron level at which magnetic memory elements are being manufactured. It would be advantageous to be able to induce magnetic anisotropy in layers of a magnetic memory element by methods other than photolithography, which methods are accurate and reproducible at the sub-micron level of production.